Generally, fossil fuels are burned to produce energy for thermal power plants and industrial complexes, and also are burned in incinerators to decrease the volume of waste and to increase the chemical stability thereof. In such cases, various hazardous flue gases, such as carbon dioxide, sulfur dioxide (SO2), nitrogen oxides (NOx), dioxins, volatile organic compounds, heavy metals, etc., are produced. Among these flue gases, nitrogen oxides function as an environmental pollutant, which is harmful to the human body and causes photochemical smog or decreases a visibility distance. Nitrogen oxides are composed mainly of nitrogen monoxide (NO) and nitrogen dioxide (NO2), in which NO constitutes 95% of total nitrogen oxides. As such, nitrogen oxides are classified into thermal NOx, prompt NOx, and fuel NOx, depending on the formation procedure thereof.
In the case of a combustion boiler using fossil fuel, the fuel is pre-treated or combustion conditions are improved to reduce the emission of nitrogen oxides. However, in the interest of economy and efficiency, a combustion post-treatment process requiring an additional treatment procedure after the combustion is effective. Such combustion post-treatment includes catalytic cracking, selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), non-selective catalytic reduction (NSCR), and plasma treatment. Presently, SCR using an ammonia reducing agent is known to be the most effective.
Through ammonia-based SCR, on the surface of a denitrification catalyst, reactions take place as represented by Reactions 1 to 5 below:4NO+4NH3+O2→4N2+6H2O  Reaction 12NO2+4NH3+O2→3N2+6H2O  Reaction 26NO+4NH3→5N2+6H2O  Reaction 36NO2+8NH3→7N2+12H2O  Reaction 4NO+NO2+2NH3→2N2+3H2O  Reaction 5
Although the denitrification catalyst useful in the ammonia-based SCR may be variously prepared, a denitrification catalyst comprising a titania support, vanadium, and optionally tungsten exhibits the greatest efficiency at present and thus has been commercialized and is used all over the world. The V/TiO2 catalyst typically has high activity at 300˜400° C., but is decreased with respect to nitrogen oxides removal efficiency due to low activation energy at temperatures lower than the above temperature. On the other hand, at temperatures higher than the above temperature, the ammonia reducing agent is oxidized, thus the stoichiometric ratio of the reaction becomes inappropriate, undesirably decreasing the efficiency. Further, the thermal fatigue of the catalyst is increased, leading to a shortened catalyst lifetime.
In the flue gas, water and sulfur oxides are generally present. Such materials function to produce an ammonium salt on the denitrification catalyst, lowering the activity of the catalyst. The reactions poisoning the catalyst progress according to Reactions 6 to 8 below:2SO2+O2→2SO3  Reaction 6NH3+SO3+H2O→NH4HSO4  Reaction 7SO3+H2O→H2SO4  Reaction 8
Sulfur trioxide produced in Reaction 6 is formed into sulfate in Reaction 7, which is not decomposed on the surface of the catalyst but remains thereon, thus poisoning the catalyst. In addition, sulfuric acid produced in Reaction 8 corrodes a catalyst bed and equipment in the subsequent stage of the system.
Because the production of sulfur trioxide according to Reaction 6 actively proceeds at high temperatures, the development of a catalyst capable of realizing excellent SCR of nitrogen oxides at low temperatures has been required to minimize the above production so as to reduce the formation of sulfate and sulfuric acid in Reactions 7 and 8.
In addition, another important cause of inactivation of SCR in the low temperature range is ammonium nitrate formed at a low temperature of 200° C. or less through the reactions represented by Reactions 9 and 10 below:2NH3+2NO2→NH4NO3+N2+H2O  Reaction 92NH3+H2O+2NO2→NH4NO3+NH4NO2  Reaction 10
Ammonium nitrite (NH4NO2) produced in Reaction 9 is very unstable, and thus is decomposed at 60° C. or higher, not causing a large problem. However, ammonium nitrate (NH4NO3) produced in Reaction 10 must be considered because it has a melting point of 170° C. Reactions 11 to 13 below show the decomposition of solid ammonium nitrate:NH4NO3(s)←→NH3+HNO3  Reaction 11NH4NO3(s)→N2O+2H2O  Reaction 122NH4NO3(s)→2N2+O2+4H2O  Reaction 13
Generally, for efficient SCR in the presence of the V/TiO2 catalyst, since the temperature of the flue gas should be maintained high in the range of 300˜400° C., a process capable of supporting SCR is limited in the flue gas disposal process. For example, in the case of thermal power plants using coal or oil, the temperature after the economizer of a boiler may be a high temperature of about 350° C., and thus the SCR system can be installed. However, due to dust and/or sulfur dioxide having high concentration, the active sites of the denitrification catalyst may be lessened, and also the denitrification catalyst may be abraded. Further, ammonium sulfate may be formed in equipment in the subsequent stage of the system due to the oxidation of sulfur trioxide (SO3), leading to corrosion of such equipment. Accordingly, methods of installing the SCR system downstream of a dust collector and a desulfurization system have been proposed. However, because the temperature of the flue gas decreases considerably while it passes through the dust collector and desulfurization system, there is the need for a denitrification catalyst suitable for use in SCR even at such low temperatures. Particularly, in the system for wet flue gas desulfurization (WFGD), no SCR is expected because the temperature of the flue gas is reduced to 100° C. or less, thus an additional reheating system is required. In order to increase the actual temperature of the flue gas by about 100° C., a large amount of power, corresponding to about 5˜10% of the total power capacity of the power plant, is known to be consumed. In this way, flue gas denitrification using a conventional commercially available V/TiO2 catalyst is a process that consumes a large amount of energy depending on the high-temperature activity of a denitrification catalyst. Moreover, with the intention of obtaining predetermined efficiency at low temperatures using the high-temperature catalyst, the catalyst should be provided in a larger amount. That is, when the amount of the catalyst is increased, not only the catalyst cost but also the costs related to a catalyst reactor, a duct, the amount of reducing agent and the reducing agent supply are increased, and as well, the pressure loss of the flue gas is increased, negatively affecting the total system.
The high-temperature operation facilitates thermal fatigue of the catalyst bed, undesirably shortening the lifetime of the catalyst. Further, since sulfur dioxide is highly oxidized, ammonium sulfate, acting as a cause of corrosion, is formed in equipment in the subsequent stage of the system for SCR. In addition, even though ammonium nitrate in addition to ammonium sulfate is formed on the surface of the denitrification catalyst, a denitrification catalyst capable of decomposing such materials at temperatures lower than a conventional temperature through the catalytic cracking reaction is required. Such properties depend on the excellent oxidation and/or reduction of the denitrification catalyst. That is to say, in order to solve problems related to economic benefits, inhibition of catalytic poisoning material and extension of lifetime of the catalyst, a low-temperature denitrification catalyst having higher denitrification performance even at a low temperature of 250° C. or less is required, unlike when using the conventional V/TiO2 catalyst.
Meanwhile, manganese oxides of NMO function to easily induce the circulation of oxidation and reduction, and the oxidation state of the manganese ion thereof is readily changed, and thus the NMO may be applied to a variety of fields such as denitrification, ammonia oxidation, VOC removal, CO oxidation, etc. In addition, manganese oxides are known to have very high activity upon oxidation, and such activity may be easily understood through the change of oxidation state of manganese oxides in the gas-solid reaction. Recently, it has been reported that pure MnO2 and manganese oxide supported on alumina exhibit very high activity upon the SCR reaction using ammonia as a reducing agent in the temperature range of 380˜570 K (L. Singoredjo, R. Kover, F. Kapteijn and J. Moulijn, Applied Catalysis B: Environ., 1, 297 (1992))
In regard to conventional NMO techniques for removing nitrogen oxides, Korean Patent Laid-open Publication No. 1998-086887 discloses the use of NMO to remove nitrogen oxides at low temperatures of 130˜250° C. and to reduce unreacted ammonia emission through oxidation. According to Korean Patent Laid-open Publication No. 2002-0051885, NMO is heat treated at 300˜400° C., or supported with one, two or more metal oxides selected from among tungsten (W), iron (Fe), copper (Cu), vanadium (V), zirconium (Zr), silver (Ag), cerium (Ce), platinum (Pt), and palladium (Pd) and is then heat treated at 300˜400° C. to remove nitrogen oxides. In addition, according to Korean Patent Laid-open Publication No. 2000-0031268, sulfur oxides and nitrogen oxides may be simultaneously removed using a continuous fluidized-bed reactor in the presence of NMO (pyrolusite, β-MnO2). Further, according to U.S. patent application Ser. No. 732,082, with the aim of realizing the low-temperature activity of a V/TiO2 catalyst, based on the weight of supported vanadium, the sum of V+4 and V+3, represented by V+x (x≦4), should be 34 atoms/cm3·wt % or more, and the sum of Ti+3 and Ti+2, represented by Ti+y (y≦3), should be 415 atoms/cm3·wt % or more.
The emission source, such as an incineration facility, discharges a large amount of dioxins, in addition to nitrogen oxides, dioxins being the most poisonous material among materials known to date and having toxicity ten thousand times greater than the toxicity of potassium cyanide. Furthermore, dioxin is nobiodegradable, stable even at 700° C., and accumulates in the body and thus negatively affects the human body, causing side effects too numerous to be completely listed, and including cancers, reproduction toxicity, malformation, liver toxicity, thyroid gland disorders, cardiac disorders, etc.
Accordingly, with the aim of reducing the emission of such dioxins, there are exemplified control methods before incineration, including preliminary removal of a dioxin precursor and uniform supply of waste, control methods during incineration, including suitable operating temperature and resident time, combustion air and mixing, minimization of fly ash particles and control of temperature of flue gas, and control methods after incineration, including a combination of a wet washer, a dry washer, an activated carbon sprayer, a dust collector, SNCR, and SCR devices. In the control method before the incineration, the composition of waste is analyzed, relationships with harmful material generated after combustion are investigated, the type of waste functioning as a main pollutant is preliminarily sorted, the amount and size of waste to be added to the incinerator are maintained constant, and the composition, heat value, water content, and volatile component content are made constant, thus uniformly maintaining the combustion environment in the furnace.
The control method during the incineration controls the 3 Ts, that is,
1) temperature of 850° C. or more
2) time of 2 sec or longer
3) turbulence due to the geometry of the incinerator and second air injection.
That is, such combustion conditions are efficiently controlled and maintained, and therefore non-burned carbon or hydrocarbon in the combustion gas, in particular, a precursor capable of being easily converted into dioxins, for example, chlorobenzene or polychlorinated biphenyl, may be produced in a smaller amount.
In the control methods after the incineration, a method of adsorbing a pollutant using an activated carbon and then continuously passing it through an oxidation catalyst bed to control it is most effective. However, due to the problems with techniques and performance of the catalyst bed, the control of dioxin is mainly dependent on adsorption using an adsorbent, such as activated carbon and calcium hydroxide. Moreover, the reproduction or disposal of the adsorbent having dioxin adsorbed thereon is technically difficult, and thus an economic burden may be imposed. That is, presently available methods of adsorbing dioxin in the flue gas using activated carbon suffer because the activated carbon on which dioxin is adsorbed is difficult to reproduce or dispose, and thus economic benefits are negated.
Typically, as a catalyst for the oxidation of dioxin as a volatile halogen organic material, a catalyst obtained by adding WO3 or MoO3 to V2O5/TiO2, such as an SCR catalyst, is useful although the amount thereof must be varied for optimal activity. Like nitrogen oxides, dioxins need to be removed at temperatures as low as possible. This is because requiring less energy for heating the combustion flue gas is economically preferable, and materials decomposed by the catalyst may be undesirably re-synthesized at 205° C. or higher in the presence of oxide of metal (Cu, Fe, etc.).